This section comprises a slight revision of our recent paper (with J. R. Busch), cited herein as Reference (10), which introduces some of the concepts and features of the present invention.
The possibility of making an integrated optical correlator was first suggested by Schubert and Harris.sup.(1) in 1968. In its simplest form, a correlator consists of two spatial light modulators (SLM), traversed by an optical beam, together with some means of translating the signal modulating one of the SLM's transversely to the beam. The amount of light passing both modulators, integrated over the beam width, then varies with the position of the translating signal according to the correlation of the two modulating signals. We call the SLM with the spatially translating modulation signal the "signal" modulator, and the other SLM, which serves to analyze (or filter) the output of the first SLM the "filter" modulator or simply the "filter". All of the components needed to construct a correlator are available in present-day integrated optics technology with the exception of a conveniently programmable filter modulator. In particular, surface acoustic wave (SAW) transducers are available that have sufficient bandwith and efficiency to serve as a signal modulator; the translation of the input signal is then accomplished by the propagation of the SAW. The first step toward realization of a programmable binary (i.e., two-state) filter is the fabrication and testing of a static filter.
The static binary filter 24A consists of a segmented surface grating.sup.(2) operating in the Bragg regime. A broad beam of light 22 incident upon the grating 24A at the Bragg angle is deflected by 2.theta..sub.B in the segments in which the grating exists (binary "ones") and is undeflected in the region where the grating is absent (binary "zeros"). The incident beam is thus diffracted into two beams separated by the angle 2.theta..sub.B. Each beam is amplitude modulated: the diffracted beam according to the pattern of ones and zeros in the segmented grating, at 100% modulation; and the transmitted beam according to the ones complement of this pattern and generally with less than 100% modulation (if the grating segments are less than 100% efficient). The operation of the filter modulator has been investigated in conjunction with a signal grating which is produced by a surface acoustic wave (SAW) transducer driven by a digitally-modulated r.f. signal, thus forming a correlator. The gratings are designed such that the presence of a "one" in both the signal and reference plane results in the deflection of one bit's worth of light into the detector. Any other combination will result in no light at the detector. The correlation signal appears at the detector as the digital SAW pattern moves past the stationary filter pattern.
As shown in FIG. 10, the filter is composed of a permanent photo-resist surface grating 24A on a Ti-infused LiNbO.sub.3 waveguide 25. It is formed by first exposing the resist to the interference pattern formed by two 4880 A laser beams and then further exposing the resist through a bit mask before development. The signal is a pulse modulated 270 MHz surface acoustic wave 21. The pulse duration of 114 nsec is chosen to match the 400 .mu.m segment length of the permanent filter 24A. The autocorrelation response of the filter to the 17-bit word 10001001011101101 agreed approximately with the expected autocorrelation signal. (Similarly to FIGS. 7 and 8.)
To generate the above pattern, a 400 .mu.m segment length was used. This requires a 6.8 mm-wide light beam to illuminate a 17-bit word. To increase the word length without increasing the beam width requires a decrease in segment length. Without modifying the available equipment, we showed that 200 .mu.m segment lengths can be used with no decrease in efficiency. Problems in maintaining a square r.f. pulse prevented us from attaining our goal of 100 .mu.m segment lengths.
In the present experiment the filter and the signal grating periods were .LAMBDA..sub.f =6.8 .mu.m and .LAMBDA..sub.s =13.3 .mu.m, respectively. The different grating periods were used so that only doubly-diffracted light would enter the detector. This results in good signal discrimination even if the gratings do not have very high diffraction efficiency. However, this type of correlator has a serious flaw. If the signal and filter patterns are identical, the output is maximum, as desired. But, adding additional "1"s (i.e., additional grating segments) to the signal grating will not decrease the correlation maximum. This situation arises because "0"s are, in essence, ignored in forming the correlation, that is, no weight is given to correct occurrence of a "0" in each grating. There are a number of ways to correct this problem, all of which involve some sacrifice of signal discrimination unless high (near unity) diffraction efficiencies are achieved. The simplest solution is to redefine the coding for the second grating so that a "1" corresponds to the absence of a grating segment and a "0" corresponds to the presence of a grating segment. Then, the singly-diffracted beams are detected. If the gratings have the same period, then singly-diffracted (0-0 and 1-1 coincidences) light passes in one direction, while undiffracted and doubly-diffracted light passes in a direction 2.theta..sub.B away. Now excess "1"s in the signal do cause a decrease in correlation signal. Furthermore, if both beams are detected, they can be subtracted to effect a penality for 0-1 and 1-0 coincidences. The loss in signal discrimination mentioned above occurs when the diffracted efficiencies of the gratings segments are low enough to allow significant zero-order energy to be transmitted through a grating segment that should be diffracting all of the light incident upon it into the first-order direction.
Having demonstrated that the digital integrated optical filter is a realizable device, it is reasonable to consider how a programmable filter could be constructed. One of the criteria for the programmable device is that it should operate on voltages low enough to be compatible with semiconductor logic. This can be achieved in the manner of the large-angle optical waveguide switch first implemented by Verber et al.sup.(3) and reported on in a later version by Kotani et al..sup.(4) These devices use the small deflection of a low-voltage electrooptic deflector to bring a light beam into Bragg incidence on a fixed phase grating. The fixed grating then imparts the required larger angular change. A programmable filter using this effect could consist of a row of N-shaped electrooptic deflectors.sup.(5) followed by a single permanent grating which extends across the entire width of the beam. Energizing a single deflector would bring a segment of the light beam into Bragg incidence upon the fixed grating which would then impart the required larger deflection. The set of "one" beams and the set of "zero" beams thus generated could then be used in the type of correlator already discussed. Alternatively, a larger Bragg angle could be used to further separate the two sets of beams and the "one" beams could be used as the input to a Fourier transform correlator of the type suggested in Reference (1).
An alternative approach to the implementation of a programmable spatial filter is to use a series of individually controlled electrooptic gratings.sup.(6) as shown in FIG. 1. This approach is superior to the use of N-shaped deflectors in that a fixed grating is not required to achieve suitably large deflections. In addition, optical losses due to fringing effects are expected to be smaller in this case than for the N-shaped deflectors. In the design shown in FIG. 4, one of the gratings is an acoustooptic one, as in the present experiments, while the other is the segmented electrooptic grating that can be addressed electrically for programming. The electrooptic grating is designed to have the same period as the acoustic grating so that the ones-complement coding can be used for one of the gratings as discussed above.
Typical apparatus according to the present invention for receiving light entering therein and controlling the directions in which portions of the light travel therethrough, comprises input means for directing portions of the entering light in a predetermined input direction into a processing region in a waveguide, control means for temporarily and separately changing the index of refraction in each of a plurality of subregions in the processing region, to modulate the light travelling thereto in approximately the predetermined input direction differently from any light travelling thereto in order input directions, and output means for receiving portions of the light travelling beyond the subregions in at least one selected output direction and for responding thereto. Typically each subregion is such that providing altered indexes of refraction therein can form in the subregion a Bragg grating positioned with a direction of Bragg incidence approximately in the predetermined input direction. The control means typically comprises an electrooptic grating (having a plurality of interdigital electrodes) in each subregion, and means for applying a potential difference to each grating separately, and thus providing an electrical field in each subregion at selected times.
Typically, successive subregions are in close proximity and all of the digits in all of the electrodes therein are parallel. An electrode in each subregion typically is connected to an electrode in each adjacent subregion, typically with alternate digits throughout the processing region connected together to form one common electrode shared by all subregions. Typically the other digits in each subregion are connected together to form another electrode for that subregion only, and are insulated from all other electrodes.
Typically the input means comprises means for directing light of known or controlled intensity approximately in the predetermined input direction into the processing region, the control means comprises means for applying a separate potential difference to the electrodes of each subregion, and the output means comprises means responsive to light travelling beyond each subregion in at least one selected output direction for providing a separate indication substantially simultaneously with the indications for the other subregions.
Where the control means comprises means responsive to digital information, the output means typically comprises means for providing selectively either a first indication or a second indication in each of a plurality of indicators. Typically each indicator comprises a separate area in a tangible medium, the first indication comprises a first condition provided in an area by the output means, and the second indication comprises a second condition either provided by the output means or comprising a preexisting condition permitted to continue by the absence of any action on the area by the output means. Typically the output means, selectively for each separate area, either provides a predetermined mark therein or permits the area to remain free of such a mark. Typically the output means, selectively for each separate area, either changes a chemical, electrical, or magnetic property therein or permits the property to remain in a preexisting state. For example, it may either substantially remove an existing electrical charge therein or permit the charge to remain substantially undiminished.
Where the control means comprises means responsive to analog information, the output means typically comprises means for providing selectively, in each of a plurality of indicators, an indication responsive to a quantity that is a function of the analog information. Typically each indicator comprises a separate area in a tangible medium, and the output means affects a condition therein. Typically the output means, selectively for each separate area, determines the magnitude of a condition therein, such as by providing a controlled value of a light responsive property therein. Typically the output determines the visible shade of each area. Typically the output means determines the magnitude of a chemical, electrical, or magnetic property in each area. For example, it may determine the polarity and magnitude of any electrical charge in each area.
Typically a selected output direction is approximately twice the Bragg angle away from the predetermined input direction. Another typical selected output direction is approximately the same as the predetermined input direction. So, commonly a first selected output direction is approximately twice the Bragg angle away from the predetermined input direction and a second selected output direction is approximately the same as the predetermined input direction. Typically the input means comprises means for directing portions of the entering light to the processing region in directions related to their respective directions of entry into the apparatus.
In typical apparatus according to the invention, the output means comprises means for selectively either permitting light travelling in a selected output direction to continue in approximately the same direction or causing it to travel further in the other selected output direction. Typically the output responsive means comprises output control means for providing selectively and separately in each of a plurality of output responsive subregions; each receiving light travelling from each subregion, respectively, of the processing region; a change in the index of refraction, to modulate the light travelling thereto in approximately the first selected output direction in a first manner, and to modulate the light travelling thereto in approximately the second selected output direction in a second manner. Typically each output responsive subregion is such that providing altered indexes of refraction therein can form in the subregion a Bragg grating positioned with a first direction of Bragg incidence approximately in the first selected output direction and with a second direction of Bragg incidence approximately in the second selected output direction; and the output control means typically comprises means for providing an electric field in each output responsive subregion at selected times.
Typical output control means comprises a surface acoustic wave transducer and means for providing alternating electrical energy to the transducer. Other typical output control means comprises a plurality of electrooptic gratings and means for applying a potential difference to each grating separately.
Typically the processing subregion control means comprises means responsive to an ordered first set of separate electrical signals, the output subregion control means comprises means responsive to an ordered second set of separate electrical signals, and the output responsive means comprises also means responsive to light travelling beyond the output responsive subregions to provide an output signal that is responsive to the degree of similarity between the first and second sets of electrical signals; and the output responsive means comprises means for providing a discernible indication when the first and second sets of electrical signals are identical in all relevant properties. Typically the electrical signals are provided responsive to digital information and the discernible indication is provided by the output responsive means when the digital information represented by the first set of electrical signals is identical to the digital information represented by the second set of electrical signals.
In other typical apparatus according to the present invention, the input means comprises means for controlling separately at a predetermined or detectable instant the input direction in which a major portion of the entering light is travelling when it reaches each subregion, and the input direction controlling means typically comprises means for directing portions of the light entering the apparatus in a predetermined direction, selectively, in an input direction that is either the same as the entering direction or a different direction, one of the selectable directions being the predetermined input direction. Typical input direction controlling means comprises a surface acoustic wave transducer and means for providing alternating electrical energy to the transducer. Other typical input direction controlling means comprises a plurality of electrooptic gratings and means for applying a potential difference to each grating separately. The input direction controlling means typically comprises means for directing a major portion of the entering light selectively either in the predetermined input direction or in a different input direction.
Typically the control means is separately responsive to light in first and second predetermined input directions, and to other input directions, and the input direction controlling means comprises means for directing a major portion of the entering light selectively either in a first predetermined input direction, in a second predetermined input direction, or in a different input direction. Advantageously the first and second input directions and the subregions are so arranged that when a Bragg grating is formed in a subregion it has a first direction of Bragg incidence approximately in the first input direction and a second direction of Bragg incidence in the second input direction. Typically the input direction controlling means comprises first and second surface acoustic wave transducers and means for providing alternating electrical energy to each transducer in reponse to binary information, where a binary "zero" causes the energy to be directed to the first transducer and a binary "one" causes the energy to be directed to the second transducer.
Typically the input direction controlling means comprises means responsive to an ordered first set of separate electrical signals, the subregion control means comprises means responsive to an ordered second set of separate electrical signals, and the output responsive means comprises means responsive to an output signal that is responsive to the degree of similarity between the first and second sets of electrical signals; and the output responsive means comprises means for providing a discernible indication when the first and second sets of electrical signals are identical in all relevant properties. Typically the electrical signals are provided responsive to digital information and the discernible indication is provided by the output responsive means when the digital information represented by the first set of electrical signals is identical to the digital information represented by the second set of electrical signals.